The only way I see that happening in x-men's current cinematic run is if Rachel is not at all related to Scott or Jean (their dead), and then there's the whole time travel thing... I don't know. Fox already screwed up the pheonix saga (according to fans), I wouldn't wanna see another great story arc screwed up.

Out of all potential future X-Men movie storylines, this would be a goer. I don't believe that Fox would want to lose the draw card that is Hugh, so Wolverine would fit in. Rachel? Mayyyyybe. Jean could of got knocked up and fostered her out or something like that.

I see a Days of Future Pastesque movie happening,
but not really,
My idea for X-Men 4:
Cable goes back in time to stop the events of X-men 3 from happening.
He has to save Charles Xavier, stop his mother the Phoenix, and save his father, Cyclops.
It makes sense since Cable and the X-Men have time travel stories in the comics.
In his future Dark Phoenix' destruction forced the world to look at the serious threat of mutants. Mutants were locked in camps, sterilized, with their powers used as labor, until they were killed.
Cable's mission could not only take him to events during X-Men 3, but also into events from X-Men 2, or perhaps 1 but not likely from X-men 1.
Maybe even events from X-men First Class. (But it could be better to keep him limited to one film, like X3).
Cable travels around trying to undo the damage of X-Men 3, while another enemy (perhaps his clone Stryfe) is sent to make sure things stay the way there are.

I'm still working on the idea, right now its a little random and rambling in my mind.
But I think the way to fix what X3 did is to bring in Cable,
make it a movie about Cable, Scott Summers, maybe Stryfe and Mr. Sinister, and Jean.
This could lead into a solo franchise staring Cable, create a franchise for Cyclops.
The only problem I see is how he was born if his parents were killed?
My only idea for that would be that Cable was created in a lab by Sinister using DNA from his parents bodies.

Cable movies could also be used to fix problems in other X-Men films, as well as plot holes or mess ups like Sabertooth being different in the Wolverine movie and X1.

And as a franchise Cable could find out that just because he stopped his mother from killing her father, he didn't turn the future into an ultimate Utopia leading to sequels.
One Cable sequel can be Cable: X-Force.
And feature Cannon Ball, Sunspot, and others.

X-Men 4: Days of Future Past
Starts with the world in the middle of a war, humans against mutants. Not just a small battle like in X3, its a giant world war between humans and mutants on a grand scale. (expensive, maybe, but the war can be hinted at in the opening) Mutants are loosing, imprisoned in camps, forced to use their powers for labor and experimentation, and are ultimately killed.
Cable is a soldier for the mutants, he takes orders from the man once named Charles Xavier.
It is revealed that, because of Jean Grey's breakdown and becoming the Dark Phoenix and causing mass death and destruction, humanity decided as a whole to wipe out the mutants. many took refuge on Genosha, Magneto's Island, but that was wiped off the face of the earth by mutant hunting robots called Sentinels. Including Magneto, they think.
But there is finally hope, Xavier and others have created what they believe will save everyone. Forge, Xavier, hank McCoy, and others, have created a time machine.
They have determined that Jean's break down and mass destruction caused this chaos. If they were able to stop it, stop her from becoming Dark Phoenix then maybe this future coud be avoided.
(these scenes could take only a few minutes, like maybe 15-20)
Cable is sent back in time.

Do think that the "Days of Future Past" story should be brought to the big screen in some way? Rachel Summers and Bishop

Its happening now!

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FANTASTIC FOUR"This is our chance to make a difference" - Sue Storm - The Invisible Woman"He's stronger than any of us but he's not stronger than all of us" - Reed Richards - Mr. Fantastic"We could these powers to help people" - Johnny Storm - The Human Torch"You can't fix this, nobody can" - Ben Grimm - The Thing